One-N, six-membered heterocyclic carbene complexes and the remote heteroatom concept
Date
2008
Authors
Raubenheimer H.G.
Cronje S.
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Abstract
A whole library of six-membered N-heterocyclic carbene complexes of Ni(ii) and Pd(ii) were prepared by oxidative substitution. In some of these compounds the heteroatom occurs in a position distant from the carbene donor atom. Combined structural and physical data, especially 13C NMR results, indicate carbene character in such ligands. DFT quantum mechanical calculation at the RI-BP56/SVP level allowed the extraction of valuable chemical information predicting that rNHC (r = remote) ligands would bond more strongly than their nNHC (n = normal) isomers. This result is also corroborated by calculations on the metal complexes themselves. Orbital overlap (mainly σ) follows the order N2HC5 < nN1HC6 < rN 1HC6 when ligands derived from halo-imidazolium and halo-pyridinium salts are compared. In C-C coupling catalysis using Pd(ii) and Ni(ii) complexes, the simple one-N, six-membered carbene complexes are superior to simple two-N, five-membered examples but clear differentiation between nNHC and rNHC precatalysts in the former family, is not always possible. © The Royal Society of Chemistry.
Description
Keywords
Catalysis, Isomers, Ligands, Nuclear magnetic resonance, Oxidation, Quantum theory, Substitution reactions, Chemical information, Differentiation, Heteroatoms, Physical data, Precatalysts, Carbon inorganic compounds
Citation
Dalton Transactions
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